The relationship between 18 species of native unionid bivalves and the exotic zebra mussel,Dreissena polymorpha, was explored at a monitoring site in Pool 26 of the Mississippi River. From 1992 to 1993, colonization rates increased from 27% to 99.7% of unionids, and the mean number ofD. polymorphaper unionid increased from 2.2 to 37.4. The number ofD. polymorphaper unionid shell was related to shell length and varied among species. Four types of colonization patterns were found, each differing from the others in the relative number of colonizingD. polymorpha.Shell and ornamentation types were differentially colonized. Thick-shelled, ornamented species were more heavily colonized than were thin-shelled, unornamented species. Habitat alteration of the gravel bar site was profound, with the zebra mussels forming a pavement over the gravel bar. This resulted in adverse effects on the unionids. Once mussels exited the substrate, they were unable to penetrate the entangled mass of zebra mussels and pebbles. Consequently, unionids were exposed when water levels fell, and considerable mortality occurred.